Monometallic phosphide catalyst of Ni2P for Hydrogen production in acidic Solution

Nowadays, it is particularly important to develop efficient hydrogen evolution catalyst in the process of water electrolysis. In this paper, a monometallic phosphide (Ni2P@C) was synthesized through a simple one-step calcination and phosphorization process, which was a high efficient catalyst for hydrogen evolution reaction. Ni2P@C exhibits satisfactory activity with a low overpotential of 215 mV and small Tafel slope of 105 mV dec−1 at the current density of 10 mA cm−2. In addition, the catalyst displayed favorable stability for 16 h when current density maintained at 10 mA cm−2. This work provides reference for monometallic phosphates as catalyst for hydrogen evolution.


Introduction
Hydrogen production by water decomposition is an efficient and environmentally friendly method, which has been widely studied [1][2][3] . As we all know, platinum (Pt) is the most effective catalyst for hydrogen evolution reaction (HER) [4] . But, due to the high consumption and scarcity, it is restricted in the actual hydrogen production process [5] . Therefore, a great deal of researches focus on the development of effective, environmentally friendly and low consumption hydrogen catalysts. In recent years, phosphates have been drawing much attention on account of their tunable electronic structures [6] . Thereinto, nickel phosphides have been used as hydrogen evolution catalysts because of the unique electronic structures and low consumption [3] . For instance, Yang and co-workers used one-pot hotsolution colloidal synthetic method to prepare Ni 12 P 5 /CNT for HER with low overpotential of 129 mV when the current density attained 10 mA cm -2 [7] . Another example, Pan and co-workers synthesized Ni 2 P nanoparticles for HER with the small Tafel slope of 81 mV dec -1 and low initial overpotential of 70 mV [8] .
In our work, a high-efficiency hydrogen evolution catalyst was synthesized via simple hightemperature calcination and phosphating process. Thereinto, we used polyaniline (PANI) as carbon source and nitrogen source. As we all know, polyaniline is a kind of conductive polymer and has a large  [9] . Therefore, we synthesized carbon-coated nickle phosphide nanoparticles (Ni 2 P@C) by the coordination of the Ni 2+ and N atoms on polyaniline. Ni 2 P@C exhibited excellent catalytic activity and high durability.

Synthesis of Ni 2 P@C
Polyaniline was synthesized according to reference [10] . In detail, 12.5 mg nickel acetate and 100 mg PANI were mixed and dissolved in a mixture of N, N-Dimethylformamide (DMF) and water. Above the solution was heated to boiling and the solvent was removed. The resulting precursor was calcined at 950 ℃ with 5 ℃ min -1 for 2 h under the Ar. In addition, we used sodium hypophosphate as a source of phosphorus.

Results and discussion
The XRD spectrum of Ni 2 P@C is shown in Figure 1a, characteristic peak at 40.6°, 44.4°, 47.2°, 53.9°, 66.3° and 75.1° are corresponding to the Ni 2 P (PDF#03-0953) plane of (111), (201), (210), (300), (310) and (400). As sketched in TEM and HRTEM images (Figure1b and c), the catalyst of Ni 2 P@C were carbon coated nanoparticles. Thereinto, in Figure 1c, the lattice spacing was 0.22 nm, which is coincident with the (111) of Ni 2 P (PDF#03-0953). In addition, the mapping images (Figure1d-h) show that C, N, P and Ni elements were uniformly distributed in Ni 2 P@C. Figure 2a shows the Raman spectra of Ni 2 P@C, respectively, the D band and G band were based in 1354 and 1590 cm -1 with the peak's intensity ratio of 0.906 (I D /I G ). Thereinto, the D band and G band originated form a defect in the carbon lattice and the graphite carbon respectively. XPS diagram was tested to further realize the electronic structure of Ni 2 P@C. Figure2b is the Ni 2p spectrogram, it can be seen the peak at 857.3 eV in accord with Ni-P bond, which demonstrated the Ni 2 P was successfully synthesized. The peaks of 862.9 eV and 876.3 eV are characteristic peaks of Ni 2p 3/2 and Ni 2p 1/2 for Ni 2 P [11] . According to N 2 adsorption diagram (Figure 2c), the large Brunauer-Emmett-Teller (BET) specific surface area of Ni 2 P@C was 269.9 m 2 /g, and the pore diameter was 2.47 nm, which was proved to be mesoporous catalyst. Hydrogen evolution performance measurement was executed with a three-electrode system in acidic solution. The polarization curves and Tafel slope of PC, Ni 2 P@C and Pt-C are shown in Figure 3a and b. Ni 2 P@C exhibites desirable catalytic effect with the Tafel slope of 105 mV dec -1 and overpotential of 215 mV at a current density of 10 mA cm -2 . Figure 3c shows the double layer capacitance (C dl ) of Ni 2 P@C. The C dl value of Ni 2 P@C is 60.5 mF cm -2 , which indicates tNi 2 P@C possessing a large catalytic active surface area. Likewise, electrochemical impedance spectroscopy (EIS) was measured at the overpotential of 200 mV. The results are shown in figure 3d, the impedance value of Ni 2 P@C is much smaller than PC, which indicates that Ni 2 P@C has a fast-catalytic kinetics. Eventually, the durability was tested by cyclic voltammetry (CV) and chronoamperometry. As shown in figure3e, the Ni 2 P@C was stable within 16 h, indicating that the catalyst has approving stability.

Conclusion
In conclusion, we have successfully synthesized the Ni 2 P@C hydrogen evolution catalyst through a simple process of high temperature calcination and phosphating. Furthermore, Raman, XPS, N 2 adsorption and hydrogen evolution performance test proved that the catalyst shows satisfactory catalytic 4 effect (overpotential of 215 mV at current density of 10 mA cm -2 and Tafel slope of 105 mV dec -1 ) and stability (16 h) in 0.5 M H 2 SO 4 . This work provides a universal method for the synthesis of monometallic phosphide hydrogen evolution catalyst (Ni 2 P@C) by using polyaniline as carbon and nitrogen source.